Composite metal oxides such as PbTiO.sub.3 -PbZrO.sub.3, PbTiO.sub.3, PbTiO.sub.3 -PbZrO.sub.3 -La.sub.2 O.sub.3 and LiNbO.sub.3 are currently used to form ferroelectric films. Such ferroelectric films display piezoelectricity, pyroelectricity, electrostriction and electrooptic effects. Taking advantage of these effects, ferroelectric films are extensively used in ultrasonic cleaning, or as vibrating transducers in phonograph pickups, communications filters, or in sound wave delay circuits, high voltage generating transformers or as electronics devices such as infrared sensors. The films are also used in ignition devices or as various electrooptic materials.
To form a ferroelectric film on a substrate, a solution containing a compound of Pb, Zr, Ti or some other suitable element is coated on the substrate and the resulting coat is dried and fired. This method is commonly employed for the various advantages it offers, such as the ease of operation and the possibility of forming a ferroelectric film on a substrate having a complex shape.
The compounds of Pb, Zr, Ti, etc., contained in the coating solutions conventionally used to form ferroelectric films, are usually present in the form of alkoxides; however, such alkoxide compounds are prone to undergo hydrolysis and the coating solution prepared using them does not have high storage stability since precipitates are likely to occur in that solution.
In order to solve these problems, it has been proposed by JP-A-61-97159 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") that the alkoxy group in an alkoxide compound be partly replaced with a .beta.-diketone group. In addition, a coating solution that will experience less formation of precipitates has been proposed in JP-A-65-6335 where the use of a solution comprising a mixture of a lead alkoxide or a lead salt, a zirconium alkoxide, a titanium alkoxide, ethanolamine and an alcohol are disclosed. However, none of these coating solutions have practically acceptable levels of storage stability and it is strongly desired to develop a coating solution having better storage stability. Furthermore, the thickness of ferroelectric films that can be formed by a single application of the heretofore known coating solutions is not greater than about 100 nm and, therefore, in order to attain a practically acceptable film thickness, several coats must be applied in superposition which only adds to the complexity of the coating process.
The present inventors conducted intensive studies with a view to providing a highly practical coating solution for forming a composite metal oxide film capable of working as a ferroelectric film, which coating solution has better storage stability than other versions and which could produce practically thick films by a single application. As a result, the inventors found that the stated objects could be attained by preparing a coating solution from a combination of particular compounds.